Projector-type image displaying apparatus are known which enlarge and project a static image or dynamic image of a character or picture that is displayed on a compact light valve as a two-dimensional image, by a projection optical system, so as to display an image. Recently, attention has also been focused on an enlargement projection method that uses a display device (referred to as a light valve, below) which uses a transmission-type or reflection-type dot matrix liquid crystal, a DMD (Digital Micro-mirror Device) or the like, enlarges an image displayed on the light valve and projects it onto a screen so as to show a large-size image. Furthermore, as for a light valve, recently, attention is focused on an LCOS (Liquid Crystal on Silicon) that is excellent in a contrast characteristic, in addition to a transmission-type liquid crystal panel and a DLP (Digital Light Processor).
Practically, an image enlarging projection apparatus (a projector) has also been utilized widely, at an office, a school and a home, since no constraint is required for an image size so as to allow a powerful large image to be obtained.
As for a projector-type image displaying apparatus, there are provided a front-projection-type one which enlarges an image on a light valve and projects it onto a projection surface such as a reflection-type screen provided away from the apparatus so as to observe reflected light and a rear-projection-type one with a transmission-type screen provided in the apparatus as a projection surface on which an image on a light valve is enlarged and projected from the side of the back face of the screen so as to observe an image from the side of the front face of the screen.
As for an example of optical system, JP-A-2004-258620 is provided. It is a projection optical system with a configuration such that an intermediate image is once formed by a lens system and it is enlarged and projected by a concave mirror, and projection in a close range has been realized. However, since there is no mechanism for changing the size of a projected image and projection with a fixed size is only allowed, there is a possibility of restricting a use condition on the condition that a projection size is fixed.
Therefore, optical systems for changing the size of a projected image have been suggested, and as an example of a projection optical system for a front-projection-type one, JP-A-2003-177320 is provided which requires a telecentric system composed of four mirrors. The projection optical system disclosed in JP-A-2003-177320 is composed of four mirror elements arranged oppositely and is a projection optical system that allows projection with a variable magnification in a close range by means of mirror movement.
However, since a mirror has a sensitivity of performance degradation to the positional displacement thereof which is generally higher than that of a lens, it is expected that the degradation of an image quality due to an error in the position of a mirror is large when the mirror is moved for projection with a variable magnification. That is, it is necessary to move a mirror for variation of magnification but it is expected that the degradation of an image quality due to an error in the position of the mirror is large and it is considered that it is necessary to provide a strict precision of the arrangement of the mirror.
Also, an optical path on which light rays are repeatedly reflected between mirrors is adopted and the light rays are high at the last mirror at the side of enlargement. Accordingly, it is difficult to reduce the height of such an apparatus and the size of the apparatus is larger in use. Furthermore, the size of the forth mirror is also large. In addition, since the mirror is provided outside a housing of the apparatus, it is considered that the mirror is easily deteriorated due to an external factor such as dust, contaminants, and impacts.
Also, JP-A-2004-295107 discloses a variable magnification projection optical system in which a first optical system composed of plural lens systems capable of moving to the side of an object and a second optical system with a mirror system having a reflective curved surface at the side of an image are arranged.
In the example of optical system disclosed in JP-A-2004-295107, variation of magnification is attained by moving a lens part while a mirror part is fixed. Also, it includes an optical system composed of lens systems and an optical system composed of plural curved mirrors and the lens systems form an intermediate image in the mirror part. The position of the intermediate image is provided at the enlargement side of the mirror system from the first surface that is the closest to the lens systems in the mirror system.
In this system, the image size of the intermediate image is changed by moving the lens part and the changed intermediate image is imaged by a second optical system again, whereby variation of magnification is attained by changing the size of a projected image. However, since the angle of view is changed for variation of magnification without a projection distance, it is necessary to change the (total) focal length of an optical component having an optical power (a lens part) by a factor of the magnification change of the size of a projected image, whereby the optical component is complex and the degree of the movement of a lens part serving to it is also large.
Also, plural rotationally asymmetric aspherical mirror is required in a practical example, in order that an aberration change caused by a change of the focal length is compensated for by the mirror part, and it goes without saying that the cost is increased. Then, the manufacturing assembly is difficult due to the high tolerance sensitivity and the projection distance, per se, is so large that it is not suitable for use in a small space like a projection optical system for a close range.
Furthermore, a variable magnification optical system in which the first optical system is composed of a transmission refractive optical system and the second optical system is composed of plural mirrors is disclosed and illustrated in JPA-2004-295107, but no example of one-mirror configuration is provided. In the second optical system for repeating reflection by the plural mirrors, the heights of light rays gradually increase while they are sequentially reflected from the plural mirrors, and therefore, it is difficult to bring the height of a mirror system in line with the height of a lens system and to be configured to a compact one. As a result, it is difficult to configure an apparatus with a small height.
Also, the first optical system is composed of plural movable lens systems, and if the lens systems are folded and configured to a compact one, a lens in front or back of folding is moved, whereby it is considered that the mechanism of a cam is complicated. In the disclosed figure, all the lens groups of the first optical system are moved for variation of magnification, and therefore, it is considered that it is difficult to configure a mechanism for folding a lens system. Also, since the full length of the part of mirror system in the second optical system is large, it is considered that it is difficult to configure the apparatus to a compact one.
Furthermore, the variable magnification optical system disclosed in JP-A-2004-295107 has a function of changing a projection magnification by changing an angle of view at a generally identical projection distance, but no function of changing a projection magnification at the time of changing the projection distance is disclosed. Also, since the angle of view is small in the variable magnification optical system, the projection distance has to be changed drastically if the projection magnification is changed by changing the projection distance while the angle of view is kept constant. Since the projection distance is changed drastically, the degree of focusing also increases.
FIG. 23 is a schematic diagram showing the configuration of an optical system disclosed in JP-A-2004-295107.
Both a first optical system 102 and a second optical system 104 form real images, and therefore, their powers are positive. Also, the second optical system 104 is composed of a mirror system but even if it is schematically expressed by a lens, there is no problem in the following descriptions. In the relation between an object 101 and an image 105 thereof, the size of an intermediate image 103 is changed in order to change the size of the image 105 since the second optical system 104 is fixed (see FIG. 2 of JP-A-2004-295107). Then, the first optical system 102 has to change the power thereof as well as to move the principal points thereof, in accordance with the size change of the intermediate image 103. Practically, the focal length or magnification has to be changed by a factor of the magnification change of the intermediate image 103 in the paraxial theory. That is, where a factor of the magnification change of an image size (the value of the maximum image size divided by the minimum image size) is represented by α′ and the maximum focal length and minimum focal length of the first optical system 102 are represented by fa′ and fb′, respectively, the following formula (2) is satisfied.α′=fa′/fb′  (2)